Portable and integrated microfluidic flow control system using off-the-shelf components towards organs-on-chip applications

Organ-on-a-chip (OoC) devices require the precise control of various media. This is mostly done using several fluid control components, which are much larger than the typical OoC device and connected through fluidic tubing, i.e., the fluidic system is not integrated, which inhibits the system’s portability. Here, we explore the limits of fluidic system integration using off-the-shelf fluidic control components. A flow control configuration is proposed that uses a vacuum to generate a fluctuation-free flow and minimizes the number of components used in the system. 3D printing is used to fabricate a custom-designed platform box for mounting the chosen smallest footprint components. It provides flexibility in arranging the various components to create experiment-specific systems. A demonstrator system is realized for lung-on-a-chip experiments. The 3D-printed platform box is 290 mm long, 240 mm wide and 37 mm tall. After integrating all the components, it weighs 4.8 kg. The system comprises of a switch valve, flow and pressure controllers, and a vacuum pump to control the diverse media flows. The system generates liquid flow rates ranging from 1.5 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}μLmin\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{-1}$$\end{document}-1 to 68 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}μLmin\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{-1}$$\end{document}-1 in the cell chambers, and a cyclic vacuum of 280 mbar below atmospheric pressure with 0.5 Hz frequency in the side channels to induce mechanical strain on the cells-substrate. The components are modular for easy exchange. The battery operated platform box can be mounted on either upright or inverted microscopes and fits in a standard incubator. Overall, it is shown that a compact integrated and portable fluidic system for OoC experiments can be constructed using off-the-shelf components. For further down-scaling, the fluidic control components, like the pump, switch valves, and flow controllers, require significant miniaturization while having a wide flow rate range with high resolution. Supplementary Information The online version contains supplementary material available at 10.1007/s10544-023-00657-z.

1 Flow control design

Stable vacuum subsystem
The stable vacuum subsystem generates the necessary pressure difference for fluid flow in the system.
The sequence of operation is given below.
(1) Open the normally-closed shut-off valve (24 V) to the flow control subsystem.
(2) Switch-on the vacuum pump (12 V) for 5 s. (3) Close the shut-off valve to seal the vacuum inside the waste reservoir. Applying 12 V to the vacuum pump ensures a maximum vacuum of 610 mbar below atmospheric pressure in the reservoir commensurate with the performance of the vacuum pump used. The vacuum in the reservoir can be maintained by opening the shut-off valve and keeping the pump running at 5 V instead of 12 V. This low voltage significantly minimizes the pump vibrations, noise and power consumption. Periodically (every 10 min for 5 s) operating the pump at a higher voltage compensates for the vacuum loss with time.

Flow control subsystem
The flow control subsystem steers the fluid and controls the flow rate through the OoC. The flow rate in the flow control subsystem as described in Fig. 2 is given as where ∆P const is pressure difference generated in the waste reservoir created by the vacuum pump, R T ube , R Switch , R Splitter and R Cont are hydraulic resistances of the connecting tubes, switch valve, 3-way valve splitter and tubing inside the flow controller respectively. Two Cori-FLOW mass flow controllers (R Cont ) from Bronkhorst High-Tech B.V. controlled the fluid flow. They use the Coriolis principle to measure the flow rate. The desired flow rate is programmable in the microcontroller connected to the flow controllers.
A switch valve and a 3-way valve controlled multiple fluids through the OoC chip (Fig. 2). The IDEX switch valve used had six fluid inlet channels and one fluid outlet channel. Each channel was connected to different fluids. By applying 4-line BCD control signals, the stepper motor in the valve rotated and connected to the desired fluid position. The microcontroller reads and controls valves' status (ON or OFF). The 3-way valve ensures the same liquid in both cell chambers, e.g., DI water or ethanol, to initialize the chips. After initialization, the valve separated flow in the top and bottom cell chambers. Flow path 1 allows gas or liquids from tubes B to F and flow path 2 allows a constant liquid flow from tube A. The flow path 1 is connected to the epithelial cell chamber and flow path 2 to the endothelial cell chamber for lung-on-a-chip application.

Cyclic vacuum subsystem
Stretching of the membrane separating both cell chambers by cyclic vacuum (≈ 0.2 Hz) in the side chambers emulated the breathing function of a lung-ona-chip. As shown in Fig. 2, the side channels can switch connections between vacuum pump or atmosphere with a T-junction. The flow path to the atmosphere has a flow controller controlling the amount of vacuum in the side channels. The amount of vacuum needed, the stretching frequency, and the waveform shape is programmable in the microcontroller. The pump can create a vacuum of at least −500 mbar in the side chambers.              Supports used to separate top and bottom plates. The third picture shows the middle support which is also used to form a window for chip accommodation. The square hole in first picture is used to connect Arduino microcontroller to outside laptop and make room for battery connection and its switch. The circle hole is used to accommodate the switch for the entire system. Support frame for fixing the gauge. The curve part is used to fit the splitter connected to gauge and the structure is fixed by double-side tape in curvature.
The frame used to fix switch valve. The valve is vertically placed and two UNC 4-40 screws are used to fix the switch valve through the holes on top of the frame.
The frame used to fix vacuum pump. The pump has a cylinder structure thus can be fixed by double-side types in the curvature part. Holes are used to fix the frame on top of the top plate.
The support used to place the manual valve at higher location to decrease the stress in vacuum tubes. The structure are all connected by double-side types.